This invention relates to an electronic fuel injection control system for internal combustion engines, and more particularly to an electronic fuel injection control system of this kind which is adapted to increase the quantity of fuel being supplied to an internal combustion engine after the start of the engine, so as to obtain stable operation and enhanced driveability of the engine.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
On the other hand, it is generally employed to increase the quantity of fuel being supplied to the engine after the start of the engine, to prevent engine stall after the start of the engine.
However, in carrying out this after-start fuel increase by the use of the above-mentioned proposed fuel supply control system having the electronic computing means, there is the following problem: An electronic control unit (hereinafter called "ECU") containing a central processing unit (hereinafter called "CPU") is generally used as the above electronic computing means. Usually, the ECU is supplied with operating voltage from a battery installed in the automotive vehicle on which the engine is installed, which battery is also used as a power source for other electric parts of the automotive vehicle. The starting motor of the engine which forms one of the electric parts of the automotive vehicle is connected to the above battery by way of a starting switch. The starting switch is closed for a period of time from the start of the engine until after the engine has reached its complete firing state, to connect the battery to the starting motor for actuating same, and after the lapse of the above period of time, the starting switch is opened. Then, the starting motor is supplied with electric power from a charging generator whose output voltage has already been elevated by that time. However, at an instant when the starting switch is closed to actuate the starting motor at the start of the engine, there can usually occur a temporary drop in the output voltage of the battery due to its sudden feeding to the starting motor. The degree of the battery voltage drop becomes larger as the ambient temperature drops. Accordingly, in very cold weather, there occurs a corresponding temporary drop in the supply voltage of the ECU which is also fed by the same battery so that the output voltage of a voltage regulator circuit provided in the ECU, which is fed to various parts of the ECU drops below a rated voltage. Further, various sensors for sensing various operating condition parameters of the engine also undergo a drop in their supply voltage so that their analog output voltages cannot represent accurate parameter values. In addition, an A/D converter provided in the ECU for converting analog voltages outputted from the sensors into digital signals fails to function properly so that its output digital values are inaccurate. The CPU is supplied with these inaccurate digital values and also undergoes a drop in the supply voltage applied to itself below its minimum proper operating voltage, resulting in an improper arithmetic operation. As known, the after-start fuel incremental value is determined as a function of the temperature of the engine such as the temperature of engine cooling water. The engine cooling water temperature is sensed by a water temperature sensor composed of a thermistor or the like. If a digital value indicative of the sensed water temperature value, which is outputted from the A/D converter, is not accurate, the after-start fuel incremental value, which is arithmetically calculated by the CPU on the basis of the above digital value, is not correct, resulting in degraded driveability of the engine after the start of the engine.